Getting to Grips with the Simplified 4G/LTE Architecture: Your Go-To Technical Guide
The 4G/LTE architecture has changed the game for mobile broadband. It brings in an all-IP network built for fast data speeds, low latency, and smooth mobility. Check out the image labeled “Simplified 4G/LTE Architecture” — it gives a great visual overview of the LTE network’s journey, starting from User Equipment (UE) all the way through radio access, the Evolved Packet Core (EPC), and out to external IP networks and applications.
What follows is a comprehensive, easy-to-understand look at each element illustrated in the image. Whether you’re an experienced telecom engineer or just someone eager to learn about tech, this guide will clarify how LTE networks operate from a systems perspective.
- Overview of the LTE Architecture
The architecture depicted in the image breaks the system down into four main segments:
User Equipment (UEs)
Radio Access Network (RAN)
Wireless Mobile Core (EPC)
Applications (IMS, OTT services, and PDN)
This layered structure simplifies how signaling and user data flow from devices to outside services.
- User Equipment (UE)
On the left side of the image, you’ll see smartphones connecting to the eNodeB (eNB) through the LTE-Uu interface.
What UEs Do:
Start attachment procedures
Manage RRC signaling with the eNB
Send both user-plane and control-plane data
Carry out mobility measurements (RSRP, RSRQ)
The UE is essentially the entry point into the LTE ecosystem.
- The Radio Access Network (RAN)
The eNodeB stands out in the image, signified by a radio tower icon.
eNodeB (eNB) Tasks
Manages radio communication with the UE
Oversees Radio Resource Management (RRM)
Schedules and executes MAC layer functions
Sets up S1 connections with the EPC
The RAN connects to the EPC using two main interfaces:
S1-MME (control plane) → connects to the MME
S1-U (user plane) → connects to the SGW
This separation for user and control traffic is essential in LTE design.
- Wireless Mobile Core (Evolved Packet Core – EPC)
The EPC forms the backbone of LTE architecture, featuring the major components:
MME
HSS
SGW
PGW
PCRF
Each element is crucial for tasks like authentication, mobility, routing, policy enforcement, and charging.
4.1 Mobility Management Entity (MME)
In the middle of the image, the MME serves as the main control-plane node.
MME Key Roles
Authenticates with HSS via S6a
Establishes bearers with SGW
Manages EPS sessions
Handles paging and tracking in idle mode
Assigns TA lists
Controls security (key management)
The MME doesn’t deal with user data — that’s the job of SGW and PGW.
4.2 Home Subscriber Server (HSS)
Connected to the MME via S6a, the HSS acts as the main database for subscriber information.
HSS Roles Include:
Stores subscriber identities (IMSI)
Keeps authentication keys
Offers QoS and subscription profiles
Supports roaming and mobility information
In LTE, the HSS functions similarly to the HLR in 3G but operates on an IP basis.
4.3 Serving Gateway (SGW)
Below the MME, connected through S11, the SGW manages the user-plane path from UE to PGW.
SGW Key Functions
Routes packets between eNB and PGW
Acts as a mobility anchor during handovers between eNBs
Enforces QoS markers
Buffers data during inter-eNB mobility
The SGW is the first gateway for user traffic in the core.
4.4 Packet Gateway (PGW)
To the right of the SGW, the PGW serves as the exit point to the external Packet Data Network (PDN).
PGW Key Responsibilities
Allocates IP addresses
Routes data to PDNs through the SGi interface
Enforces per-flow QoS
Filters traffic (via TFT)
Monitors usage and charging
The PGW ensures each user session aligns with the policies set by the operator.
4.5 Policy and Charging Rules Function (PCRF)
Above the PGW, the PCRF oversees policy and charging decisions.
PCRF Functions
Provides QoS rules to PGW over the Gx interface
Coordinates with applications (IMS, OTT apps) via Rx
Enforces policy rules like throttling, prioritization, or zero-rating
Sets charging parameters
The PCRF is vital for service-based and differentiated charging models.
- Application Layer (IMS and PDN)
On the right side of the diagram, you’ll see:
IMS (IP Multimedia Subsystem)
Handles SIP-based services like:
VoLTE
VoWiFi
Rich Communication Services (RCS)
Apps
OTT applications connect with PCRF through Rx to impact policy decisions for specific application sessions.
PDN (Packet Data Network)
This signifies the external IP network containing:
Internet services
Corporate VPNs
Operator services
The PGW connects to the PDN through the SGi interface.
- Overview of LTE Interfaces (From the Diagram)
The image shows the main logical interfaces:
InterfaceBetweenPurposeLTE-UuUE ↔ eNBRadio access air interfaceS1-MMEeNB ↔ MMEControl-plane signalingS1-UeNB ↔ SGWUser-plane dataS11MME ↔ SGWSession and bearer controlS6aMME ↔ HSSAuthentication and subscriber infoS5/S8SGW ↔ PGWData tunneling for roaming/home scenariosGxPGW ↔ PCRFPolicy controlRxPCRF ↔ IMS/AppsService-based policy controlSGiPGW ↔ PDNData exit to external networks
Getting these down is crucial for troubleshooting and network design.
- LTE Data Flow (Based on the Image)
Step-by-step end-to-end flow:
UE sends an attach request to eNB
eNB forwards signaling to MME via S1-MME
MME authenticates subscriber with HSS (S6a)
MME instructs SGW to create a bearer (S11)
SGW talks to PGW to allocate an IP and build a data tunnel (S5/S8)
PGW asks for policy rules from PCRF (Gx)
The PCRF might consult apps/IMS for QoS (Rx)
User data flows on S1-U → SGW → PGW → SGi → PDN
This process guarantees efficient and controlled data delivery throughout the LTE ecosystem.
Table: EPC Components and Their Roles
Component Plane Function MME Control Mobility, authentication, session management HSS Control Subscriber database and authentication SGW User Packet routing, mobility anchor PGW User Internet access, charging, QoS enforcement PCRF Control Policy and charging rules IMS Service Vo LTE and SIP-based services
This breakdown reflects the structure shown in the uploaded image.
Conclusion
The uploaded Simplified 4G/LTE Architecture diagram offers a clear, high-level view of how LTE networks handle signaling and user data from mobile devices to external IP services. By getting a grasp of each component—UE, eNB, MME, HSS, SGW, PGW, PCRF, and IMS—and how they connect, telecom professionals can better visualize how LTE guarantees reliable connectivity, effective QoS management, seamless mobility, and top-notch performance.
As we move toward 5G, LTE continues to be the foundational technology that supports billions of users globally. A solid understanding of the LTE architecture is crucial for anyone involved in designing, optimizing, and troubleshooting today’s mobile networks.
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